The future exhaustion of oil resources has been concerned for a long time. During this time, in order to decrease the dependence on the oil resources even in a small degree, there have been studied technologies of producing various kinds of hydrocarbon oils such as naphtha, kerosene, and gas oil, using other carbon sources such as natural gas, coal, or biomass as a feedstock. Under such circumstances, the GTL process is considered to have reached a practical stage as a technology to a certain extent, and the operation of a plant on a practical scale has already started in a region where natural gas is produced in abundance. Similar plants will be constructed further in the future.
The GTL process is performed as follows: natural gas containing methane (CH4) as a main component is reformed, whereby a synthesis gas containing hydrogen (H2) and carbon monoxide (CO) as main components is produced; Fischer-Tropsch synthesis (FT synthesis) is performed using the synthesis gas as a feedstock, whereby so-called Fischer-Tropsch oil (FT oil) that is a mixture of various kinds of hydrocarbon oils containing heavy hydrocarbon is produced; and the FT oil thus obtained is refined by upgrading, whereby various kinds of oil products such as naphtha, kerosene, and gas oil are produced. Thus, the GTL process roughly includes three sections: a synthesis gas production section (reforming section), a Fischer-Tropsch oil production section (FT section), and an upgrading section (UG section).
In the reforming section of the GTL process, conventionally, an autothermal reforming (ATR) process or a partial oxidation (POX) process has been used in most cases. A steam reforming process or a carbon dioxide reforming process in which steam or carbon dioxide is allowed to react with the natural gas directly can be conducted more simply in terms of the principle for converting the natural gas into the synthesis gas. However, those processes are not used so often because of the following reason.
According to the steam reforming process or carbon dioxide reforming process, steam or carbon dioxide is added to the natural gas, and the mixture is allowed to pass through a reaction tube filled with a reforming catalyst. Then, the reaction tube is placed in a radiation section of a tubular reformer and heated from outside, whereby the natural gas (methane) is converted into the synthesis gas in accordance with Reaction Formula (1) or (2).CH4+H2O→CO+3H2  (1)CH4+CO2→2CO+2H2  (2)
Here, it should be noted that the molar ratio between hydrogen and carbon monoxide in the synthesis gas to be obtained can be adjusted in a range of 3:1 to 1:1 by adjusting the ratio between the Reaction Formula (1) and the Reaction Formula (2). Both the reactions are endothermic reactions, and hence, it is necessary to feed a great amount of heat from outside. Therefore, there is a problem in that the heat efficiency is low, and a reaction apparatus is enlarged if those reactions are applied to a large-scale production.
In contrast, each of the ATR, the POX, and a catalytic partial oxidation (CPOX) process that has recently reached a practical stage can be called a partial oxidation process in a broad sense. According to those processes, oxygen (instead of steam or carbon dioxide) is added to the natural gas, and the natural gas is converted into the synthesis gas in accordance with Reaction formula (3) below.CH4+(½)O2→CO+2H2  (3)
According to this reaction, in the case of the POX, the natural gas that is a feedstock is partially burnt with oxygen to generate hydrogen and carbon monoxide. This reaction is conducted without a catalyst, which requires high temperatures of 1,000° C. or more, for example, about 1,400° C. Therefore, it is necessary to add oxygen in an amount greater than a stoichiometric amount, and accordingly, the ratio between hydrogen and carbon monoxide to be generated becomes 2 or less. Therefore, the ratio of hydrogen and carbon monoxide may be adjusted to 2 by setting a small steam reformer in addition to the POX. In contrast, it is considered that the ATR includes a first stage of burning a part of the natural gas that is a feedstock by the addition of oxygen to generate water (steam) and carbon dioxide; and a second stage of allowing the generated steam and carbon dioxide to react with the remaining natural gas, thereby generating hydrogen and carbon monoxide. More specifically, it can be considered that this reaction itself generates and feeds steam and carbon dioxide used for a steam reforming process and a carbon dioxide reforming process. The first stage is a combustion reaction involving the large generation of heat, and the second stage is an endothermic reaction in accordance with Reaction Formula (1) or (2). Those stages are a heat generation reaction as a whole, and hence, it is not necessary to feed heat from outside, and there is an advantage in that the heat efficiency of the reforming section itself is high. Therefore, in the reforming section of the GTL process, those processes are often used.
The synthesis gas produced in the reforming section contains hydrogen and carbon monoxide as main components. In the FT section, FT oil is produced by FT synthesis in accordance with the following Reaction Formula (4), using the thus obtained synthesis gas containing hydrogen and carbon monoxide as a feedstock.2nH2+nCO→—(CH2)n-+nH2O  (4)
As is understood from Reaction Formula (4), stoichiometrically, it is preferable to feed hydrogen and carbon monoxide in a molar ratio of 2:1 in the FT synthesis. As described above, a synthesis gas containing this composition can also be obtained by using both the steam reforming reaction and the carbon dioxide reforming reaction. An intermediate product produced by the FT synthesis contains a gaseous product (containing unreacted hydrogen and carbon monoxide, or hydrocarbon with 4 or less carbon atoms, etc.). Therefore, in the FT section, the gaseous product is removed by separation after the FT synthesis to produce FT oil.
The FT oil obtained in the FT section contains heavy components. Therefore, in the UG section, various kinds of petroleum products such as naphtha, kerosene, and gas oil are produced by performing hydrotreatment, or hydrocracking and distillation (rectification). Typically, first, the FT oil is distilled to separate to heavy components and the other light components. Then, the heavy components are hydrotreated (including hydrocracking) to be light components, and the obtained light components are further separated by distillation. The FT oil introduced into a distillation column for the separation by distillation needs to be heated to a predetermined temperature (e.g., 300° C. to 350° C.) previously. Also, before the separated heavy components are introduced into a hydrogenation reactor for hydrotreatment, the heavy components need to be heated to a predetermined temperature (e.g., 300° C. to 400° C.). In the conventional GTL process, the separated heavy components are directly heated by a heating furnace or are indirectly heated by providing a heat exchanger for heating separately from the heating furnace and circulating hot oil between the heating furnace and a heat exchanger for heating.
In the ATR and the POX currently used in the reforming section of the GTL process, it is necessary to feed excess steam so as to keep the life of a burner, which causes a problem in that it is difficult to operate the process under economically optimum conditions. There also is a problem in the POX in that the controllability of the reaction is difficult, and a great amount of soot is likely to be generated. The CPOX that does not use burner combustion has been developed so as to solve the above problems. However, the CPOX has a problem in that heat generation is concentrated in the vicinity of the inlet of a catalyst layer and hot spots are likely to be generated. The reason for this is as follows: the combustion reaction in the first stage involving the large generation of heat precedes in the vicinity of the inlet of the catalyst layer; and a reforming reaction in the second stage involving the absorption of heat occurs toward a downstream portion. It was found that this problem could be solved by allowing the heat generation reaction in the first stage and the endothermic reaction in the second stage to proceed simultaneously in parallel, or allowing a partial oxidation reaction in accordance with Reaction Formula (3) to be directly effected. A catalyst and an apparatus therefor have been developed recently (JP 2005-193110 A, JP 2005-199263 A).